1. Field of the Invention
The present invention relates to a method for controlling a handoff in an optical distributed network system, and in particular, to a method for controlling a handoff in an optical distributed network system using multi input multi output (MIMO).
2. Description of the Related Art
Since the late 1970s the U.S. has developed a cellular mobile communication system. Korea is now beginning to provide a voice communication service by an advanced mobile phone service (AMPS) method that is based on an analogous 1st generation (1G) mobile communication system. In the middle 1990s, a code division multiple access (CDMA) system was commonly used as a 2nd generation (2G) mobile communication system. The CDMA system provides voice and low-rate data services.
International mobile telecommunication-2000 (IMT-2000), is under development is a 3rd generation (3G) mobile communication system beginning from the late 1990s with the objection of improved radio multimedia service, global roaming, and high-rate data service. The 3rd generation mobile communication system has been developed to transmit data at a higher rate to accommodate the rapid increases in the amount of service data of the mobile communication system.
As the 3rd generation mobile communication system begins to be commonly used, attention is being transferred to a beyond 3rd generation (B3G) or 4th generation (4G) mobile communication system. The B3G or 4th mobile communication systems are being standardized with an aim of creating an effective association and an integral service of a wire communication network and a wireless communication network.
Accordingly, the wireless communication network is requires a technology for transmitting a large capacity data that comes close to the capacity of the wire communication network. For this, a mobile communication system using multi input multi output (MIMO) is presently under investigation.
In general, the MIMO employs a multi transmission antenna and a multi reception antenna instead of one transmission antenna and one reception antenna, thereby improving an efficiency of data transmission. Basically, the MIMO greatly increases the data transmission efficiency by transmitting and receiving several signals using several antennas at the same time. Thus, it has an advantage of transmitting much more data than in the existing mobile communication system without increasing the required bandwidth.
There is a great possibility in which a carrier frequency for transmitting data is set to a band higher than an existing frequency of 5 GHz. The prospect is that a cell radius would be gradually reduced to keep a high data rate and the same capacity as an existing capacity according to a free space propagation model. Thus, a distributed network system based on a picocell of about one hundred meter seems to be required.
A conventional method for executing the distributed network system on a per-picocell basis uses an optical relay or multi hop technology.
The multi hop technology, a technology recently proposed for constructing a picocell having many cellular systems, can widen a service boundary of a cell without installation of a separate wire line. However, the multi hop technology has a drawback in that frequency interference occurs and thus, is limited in constructing and managing the cell.
However, the method using the optical relay has an advantage that it does not cause such a drawback and thus, is free from propagation interference in managing the picocell. Thus, in actuality, the distributed network system is using the optical relay.
FIG. 1 illustrates a construction of the distributed network system using the optical relay.
Referring to FIG. 1, the distributed network system includes a base station transceiver subsystem (BTS) 101, a base station controller (BSC) 102, a base station (BS) 103, and a radio access unit (RAU) 104.
In a detailed description of the distributed network system using the optical relay, the base station transceiver subsystem 101 performs a function of radio access with a mobile terminal (MT) (not shown), and a function of wire and radio access between the mobile terminal and the base station controller 102.
The base station controller 102 is positioned between the base station 103 and a mobile services switching center (not shown), and manages and controls the base station transceiver subsystem 101 and the base station 103.
The base station 103 connects with the base station transceiver subsystem 101. The base station 103 receives a signal from the mobile terminal provided within its managing picocell, over a wireless channel, and transmits the received signal to the mobile services switching center. Similarly, the base station 103 transmits a signal coming from the mobile services switching center, to the mobile terminal over the wireless channel.
In general, in the distributed network system using the optical relay, a large area is divided into a small area that is called picocell, for the effective use of the wireless channel. The distributed network system performs a wireless communication with the mobile terminal through the base station 103 provided in each picocell. The picocell defines a wireless coverage area established by the base station 103 positioned in each picocell. Similarly, each of the other picocells defines a related wireless coverage area established by a corresponding base station 103 positioned among the associated picocell.
The radio access units 104 connect with the base station transceiver subsystem 101, and define the picocells around the corresponding base station 103. The base station 103 and the mobile terminal perform the wireless communication with each other using the radio access units 104. Moving in position in course of the wireless communication within the picocells, the mobile terminal measures a signal strength of each picocell, and clamps to the most relevant picocell.
The most important issue of the distributed network system constructed by the many picocells is to process a handoff that is frequently implemented when the mobile terminal moves between the picocells so as to keep a state of uninterrupted communication.
The handoff refers to changing a communication path to a cell to which movement is implemented, to keep a communication when the mobile terminal moves to another base station (or sector) out of an in-service base station (or sector) in the general mobile communication system. Unlike an analogous method supporting a hard handoff where an existing communication line is first cut and then is connected to a new base station, a CDMA method supports even a soft handoff where communication paths with two base stations (or sectors) are concurrently maintained.
FIG. 2 illustrates an example of a conventional operation of processing the soft handoff in the mobile communication system.
FIG. 2 conceptually exemplifies variations of intensities of signals received from a base station1 202 and a neighbor base station2 203 when a mobile terminal 201 moves from “a” point to “b” point within an overland (OL) area where a service boundary cell 1 of the base station1 202 and a service boundary cell 2 of the neighbor base station2 203 are overlapped.
The operation of processing the soft handoff in the mobile communication system will be described with reference to FIG. 2. In this exemplary processing example, it is assumed that the mobile terminal 201 receiving a service in the service boundary cell 1 of the base station1 202 moves to the service boundary cell 2 of the neighbor base station2 203.
In a little more detailed description of the operation, the soft handoff is generated under the control of the base station controller 102 when the mobile terminal 201 is positioned in the area where the service boundary cell 1 of the base station1 202 and the service boundary cell 2 of the base station2 202 are overlapped.
In other words, the mobile terminal 201 receiving the service from the base station1 202 detects a signal strength at or exceeding a preset value (T_ADD), from the base station2 203. The detected signal strength indicates a handoff is necessary in the course of movement toward base station2 203. The mobile terminal 201 transmits the detected signal strength from base station1 203 to the corresponding base station controller 102.
In response to this, the base station controller 102 checks whether or not the base station2 203 is in an idle state. Checking in the idle state, the base station controller 102 allocates a channel between the base station2 203 and the mobile terminal 201. If the channel allocation to the base station2 203 is implemented, the base station1 202 and the base station2 203 provide services to the mobile terminal 201 over the respective allocated channels.
As the mobile terminal 201 continues to move toward base station2 203, at the signal strength received from the base station1 202 reduces below a preset value (T_DROP). In this case, the mobile terminal 201 drops the in-service base station1 202, and receives the service only from the base station2 203. The mobile terminal 201 has a time margin of a predetermined time in which base station1 202 is dropped. In other words, only when the time margin lapses after the signal strength is detected below the preset value (T_DROP) does the mobile terminal 201 drop the base station1 202. This prevents the handoff from being implemented when the signal intensity spontaneously falls.
Pilot channel information for allowing the mobile terminal 201 to measure the signal strengths received from the base stations 202 and 203 includes an active set, a candidate set, a neighbor set, and a remaining set. The active set denotes a pilot of the base station (or the sector), such as a forward traffic channel allocated to the mobile terminal 201. The candidate set denotes a pilot that is not currently the active set but is received by a sufficient intensity. The neighbor set denotes a pilot that does not currently exist at the active set or the candidate set but can become the candidate set. The remaining set denotes all pilots possible other than the above set in a current system.
As shown in FIG. 2, the soft handoff is implemented in the handoff region by monitoring the signal strength at the mobile terminal 201 of the signal strengths from the base stations 202 and 203 using several pieces of pilot channel information, and, in a predetermined critical region, generating a message that the handoff is active, and concurrently transceiving the same data from both of the base stations 202 and 203.
However, the conventional soft handoff in the mobile communication system has a drawback in that it is difficult to overcome an essential latency time caused by protocol processing between the base station controller 102 and the mobile terminal 201, in processing the handoff to be frequently implemented in a plurality of the picocells in the distributed network system for the B3C or 4th mobile communication system.